Electrical transformer



June 10, 1969 H. G. FISCHER ET ELECTR ICAL TRANSFORMER Filed March 7, 1967 I of 2 Sheet INVENTORS domes C. Schrpeder 0nd Helnz G. Fnscher ATTORNEY m m F WITNESSES Jun 10, 1969 H. G. FHSCHER ET AL ELECTRICAL TRANSFORMER Sheet Z of 2 Filed March '2, 1967 United States Patent 3,449,633 ELECTRICAL TRANSFORMER Heinz G. Fischer and James C. Schroeder, Muncie, Ind.,

assignors to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Mar. 7, 1967, Ser. No. 621,320 Int. Cl. H02h 7/14, 7/04, /00

US. Cl. 317-14 5 Claims ABSTRACT OF THE DISCLOSURE When a transformer experiences an internal electrical fault, the energy released in the fault causes a very rapid increase in the internal tank pressure of the transformer. In the prior art, this sudden increase in tank pressure is detected by a bellows type sensor disposed in the gas space above the fluid dielectric, which activates a solenoid driven seal-in relay having contacts which are used to energize alarm and protective circuits.

Since the amount of damage caused by an internal fault in a transformer is usually dependent upon the time interval between the start of the internal fault, and the time when the tarnsformer is removed from the electrical system by its protective apparatus, it is of the utmost importance that the relay which is responsive to the sensor means be extremely fast acting. Further, since it is not desirable to have nuisance tripping by the transformers protective apparatus, the relay must be very reliable, actuating the protective apparatus only upon the detection of an internal fault by the sensor means. Efforts to increase the operating speed, and thus reduce the operating time of the mechanical relay, however, result in making the relay shock sensitive, causing the relay to vibrate and momentarily close its contacts upon an accidental blow to the relay, or the cabinet within which the relay is mounted. Further, the contact bounce associated with the closing of mechanical contacts is undesirable, as it introduces a further time delay between the start of a fault and the actuation of protective apparatus.

Accordingly, it is an object of the invention to provide a transformer having a new and improved pressure responsive protective system.

Another object of the invetnion is to provide a transformer having a new and improved pressure responsive protective system which has a substantially faster response time than relay ystems of the prior art.

Still another object of the invention is to provide a transformer having a new and improved pressure responsive protective system which has a substantially faster response time than prior art relay systems, has no contactbounce, and is not shock sensitive.

A still further object of the invention is to provide a new and improved pressure responsive protective system for electrical transformers which includes a solid state relay having both normally open and normally closed contacts, which substantially eliminates the relay as a source of time delay in the operating time of the protective system, and which is not sensitive to shock and vibration.

Briefly, the invention accomplishes the above-cited objects by providing an electrical tarnsformer with a pressure sensor disposed to be responsive to the pressure in 3,449,633 Patented June 10, 1969 the air space above the liquid dielectric, or in the liquid dielectric itself, which provides a signal upon a sudden change in the pressure within the tarnsformer tank. This signal may be the closing of contacts on a bellows type pressure sensor, or a voltage signal provided by an electronic type transducer. The relay which is responsive to the signal from the pressure sensor is of solid state construction, and includes a bistable flip-flop circuit which operates static switching means. A reset switch flips the circuit into a first predetermined stable operating mode, in which the conductive side of the flip-flop circuit operates static normally closed switching means, and the non-conductive side of the flip-flop circuit is connected to operate static normally open switching means. Upon receiving a signal from the pressure sensor, the flip-flop circuit switches to the second of its two stable operating conditions, which effectively opens the static normally closed switching means, and closes the static normally open switching means. The normally open and normally closed switching means may be connected to operate alarm circuits, circuit breakers, and devices which perform other desired functions. The relay, being solid state, operates in micro-seconds, which effectively eliminates its response time from the total system operating time, and since there are no mechanical contacts to vibrate in the relay, it is essentially unaffected by shock and vibration, and has no contact bounce.

Further objects and advantages of the invention will become apparent from the following detailed description, taken in connection with the accompanying drawings, in which:

FIGURE 1 is a pictorial view of a transformer, partially cut away, illustrating an embodiment of the invention;

FIG. 1A is a fragmentary view of the transformer shown in FIG. 1, which illustrates another embodiment of the invention; and

FIG. 2 is a schematic diagram of a solid state relay constructed according to the teachings of the invention, which may be used in the protective system for the transformer shown in FIGS. 1 and 1A.

Referring now to the drawings, and FIG. 1 in patricular, there is shown a pictorial view of a transformer 10 of the type which may advantageously utilize the teachings of the invention. Transformer 10 includes a plurality of electrical coils 12 connected to provide high and low voltage windings, with the coils being disposed in inductive relation with a magnetic core 14. The magnetic corewinding assembly is disposed within a casing or tank 16, which is shown partially cut away, with the tank 16 being filled to a predetermined level 18 with an insulating and cooling liquid dielectric, such as oil. The tank 16 includes high and low voltage bushings 20 and 22, respectively, for connecting the windings with an external electrical system and electrical load, and heat exchanger means 24 for cooling the liquid dielectric.

When a short circuit or fault occurs in the electrical windings, the sudden release of energy in the fault causes a rapid increase of pressure within both the liquid dielectric, and in the air space disposed above the liquid dielectric. This sudden pressure increase, as opposed to gradual pressure changes do to ambient temperature changes and loading induced changes, is used to operate the protective apparatus associated with the transformer.

The pressure responsive protective system includes pressure sensor means, and a relay which is responsive to a signal from the pressure sensor means. The relay contains switching circuits which initiate protective and alarm circuits. Sensor means for providing a signal in response to a rapid increase in the internal tank pressure is shown in FIG. 1 at 30, and the relay may be disposed in a control panel 32, connected to the sensor means through electrical leads 34. It will be noted that in the embodiment of the invention shown in FIG. 1, the pressure sensor means 30 is disposed to be responsive to the pressure in the gas space disposed above the liquid dielectric. This may be accompanied by obtaining access to the gas space through a small opening in the tank wall, which is in sealed communication with the sensor means. Any suitable type of pressure sensor may be used. For example, the pressure sensor may be of the bellows type which contains a pressure chamber in sealed communication with the gas space above the liquid dielectric, with the bellows closing mechanical contacts upon a sudden change in tank pressure, which contacts are connected to the relay. Gradual pressure changes in tank 16, such as those due to loading factors and ambient temperatures, leak through a small opening in the pressure chamber without actuating the bellows. The opening is insufficient to dissipate a rapid pressure change, associated with internal faults within the transformer, thus moving the bellows and actuating the relay, which seals-in until manually reset.

Sensor means 30*, instead of being disposed to be responsive to the pressure in the gas space, may be disposed to be responsive to the pressure in the liquid dielectric itself, as shown in FIG. 1A. In transformers which have a gas space within the main transformer tank, disposed over the liquid dielectric, the pressure sensor is usually disposed to monitor the pressure of the gas space. In certain transformers, however, such as extra high voltage transformers (EHV), the transformers is completely filled with liquid dielectric, with a separate tank being used for expansion of the liquid dielectric due to ambient and loading factors. In this instance, the sensor would be disposed to be responsive to the pressure in the liquid dielectric. Instead of using a bellows type sensor disposed in the liquid dielectric, it would be preferable to utilize an electronic pressure transducer which provides an output voltage only when the rate of change of pressure within the liquid exceeds a predetermined value. The electronic type pressure transducer would also pro- 'vide less delay in the operation of the transformers protective system, than a mechanical bellows type system, as the speed of operating the mechanical bellows depends upon the rate of change of pressure.

The relays which are conventionally used to initiate the operation of the protective apparatus, in response to a signal from the pressure sensor means, are solenoid operated, mechanical relays, which contain contacts connected to energize the protective apparatus. Mechanical relays, in this application, have certain disadvantages. Since the internal fault in the transformer may cause extensive damage, it is important to remove the transformer from the electrical system as soon as possible, in order to minimize damage to the transformer. Fast operating mechanical relays still require one-half cycle, or more, on a 60 Hz. system, to pickup, and in addition have the disadvantage of being shock sensitive. Since it is undesirable to disconnect the transformer from an electrical system, unless absolutely necessary, the tendency of the mechanical relay to vibrate and momentarily close its contacts when subjected to shock is a distinct disadvantage.

These disadvantages are overcome, in both the embodiments shown in FIG. 1 and FIG. 1A, by using the solid state relay 100 shown in FIG. 2, which may be actuated by either the closing of contacts by a mechanical type sensor, or by a voltage signal from an electronic type transducer. The operating time of the relay is in the order of micro-seconds, and thus, for all practical purposes is eliminated from the system as far as adding to the delay of the protective system. Further, there are no mechanical contacts to vibrate and accidentally close due to shock, making relay 100 essentially shock proof. Still further, contact bounce has been eliminated.

In general, relay includes a bistable flip-flop circuit 102, first amplifier means 104, second amplifier means 106, first static switching means 108, and second static switching means 110. The static switching means 108 may be used directly to provide a normally closed contact 113- between terminals 112 and 113, as shown, which terminals are connected to protective means 116; or, static switching means 108 may be used to provide a signal for operating one of more additional static switching devices which are connected to the protective means.

Static switching means may be used directly to provide a normally open contact, in the same manner as switching means 108, or it may be used to provide gating signals for one of more additional static switching means 118. In this instance, static switching means 118 provides a normally open contact 121 between terminals and 122, and a normally open contact 123' between terminals 124 and 122. These terminals are connected to protective means 126. Protective means 116 and 126 may be alarms, circuit breakers, and any other devices which provide functions which it is desirable to activate, or deactivate, upon a fault in transformer 10.

More specifically, relay 100 includes a bistable flip-flop circuit 102, which includes first and second transistors 130 and 132, respectively. Transistors 130 and 132 may be of the PNP junction type, each having base, emitter and collector electrodes b, e and c, respectively. The emitter electrodes e of transistors 130 and 132 are connected to a conductor or bus 134, which is connected to the positive terminal of a source 136 of direct current potential, represented by a battery in FIG. 2, but which may be a bridge rectifier connected to a source of alternating potential, if desired. The negative terminal of direct current source 136 may be connected to ground 138. The collector electrodes 0 of transistors 130 and 132 are connected to bus 140 through resistors 142 and 144, respectively, and bus 140 is grounded at 138. The base electrode b of transistor 130 is connected to the collector electrode c of transistor 132 through the parallel connected resistor 146 and capacitor 148, and the base electrode b of transistor 132 is connected to the collector electrode 0 of transistor 130 through the parallel connected resistor 150 and capacitor 152.

Transistor 130 is also connected to sensor means 160, which is the pressure sensor means 30 shown in FIG. 1, and in this instance, is illustrated as being of the mechanical switch type which includes stationary contacts 162 and 164, and a movable contact 166. The base electrode 12 of transistor 130 is connected to movable contact 166 through diode 168 and capacitor 170, with the anode electrode of diode 168 being connected to the base electrode b of transistor 130. Stationary contact 162 is connected to positive bus 134, and stationary contact 164 is connected to ground 138 through resistor 172. Sensor means is illustrated in FIG. 1 with movable contact 166 on stationary contact 164, which is the normal position for sensor means 160. Upon a sudden increase in pressure, movable contact 166 will move to momentarily engage stationary contact 162.

Transistor 132 is also connected toreset means 180, which is a switch having stationary contacts 182 and 184, and movable contact 186. The base electrode b of transistor 132 is connected to movable contact 186 through diode 188 and capacitor 190, with the anode electrode of diode 188 being connected to the base electrode b of transistor 132. Stationary contact 182 is connected to the positive bus 134, and stationary contact 184 is connected to ground 138 through resistor 172.

In describing the operation of the flip-flop circuit 102, assume that transistor 132 is conducting and transistor 130 is cut-off or non-conducting. To place relay 100 in readiness for a signal from sensor means 160, transistor 130 should be conducting and transistor 132 should be cut-off. This is accomplished by moving movable contact 186 of reset means to engage stationary contact 182. Movable contact 186 should then be returned to stationary contact 184, in order to ready the reset means for the next reset operation. The return of movable contact 186 may be automatic by spring means which urges the movable contact 186 into engagement with the stationary contact 184. When movable contact 186 contacts stationary contact 182, the base electrode b of transistor 132 becomes positive, and transistor 132 switches to its non-conductive state. The collector electrode of transistor 132 thus becomes negative, which makes the base electrode 11 of transistor 130 negative, and switches transistor 130 to its conductive condition.

In the operation of relay 100, the flip-flop circuit 102 will stay in the previously described stable mode of operation, with transistor 130 being conductive and transistor 132 being non-conductive, until sensor means 160* detects a sudden increase in pressure within the tank 16 of the transformer shown in FIG. 1, at which time movable contact 166 will momentarily engage stationary contact 162. This causes the base electrode b of transistor 130 to become positive, biasing transistor 130 to cut-oft, and simultaneously making the base electrode b of transistor 132 negative with respect to its emitter electrode e, which switches transistor 132 to its conductive condition.

The switching of transistors 130 and 132 to their conductive condition is used as signals to operate static switching devices connected in series circuit relation with protective circuits, and thus functions as normally closed and normally open contacts, depending upon which end of the flip-flop circuit they are located. The output signal from transistor 130, when it is in its conductive state, is amplified by amplifier 104, which may be NPN junction type transistor 192 having base, collector and emitter electrodes b, c and e, respectively. The base electrode b of transistor 192 is connected through resistor 194 to the collector electrode 0 of transistor 130, the emitter electrode e is connected to the voltage divider provided by resistors 196 and 197, which are connected between buses 134 and 140, and its collector electrode 0 is connected to the positive bus 134 through resistor 198. Thus, when transistor 130 is conductive, the base electrode b of transistor 192 will be more positive than its emitter electrode e, biasing transistor 192 into its conductive state. When transistor 130 switches to its non-conductive state, the base electrode b of transistor 192 will be negative with respect to its emitter electrode 2, switching transistor 192 to its non-conductive state. Transistor 192, in addition to amplifying the low level output of transistor 130, provides isolation for the flip-flop circuit 102, preventing leakage current from switching means 108 from affecting the operation of the flip-flop circuit.

The output of transistor 192 is used to switch static switching means 108 to its conductive state. Static switching means 108 may be a PNP junction type transistor 200 having base, collector, and emitter electrodes b, c and e, respectively, with its base electrode b being connected to the collector electrode 0 of transistor 10-4, its emitter electrode 2 being connected to terminal 114, and to the voltage divider provided by resistors 202 and 204, which are connected between buses 134 and 140, and its collector electrode 0 is connected to terminal 112. When transistor 192 is conductive, the base electrode b of transistor 200 is more negative than its emitter electrode e, which biases transistor 200 to its conductive state, and in effect provides a closed contact for any apparatus connected to terminals 112 and 113. Thus, any apparatus which is serially connected with a direct current power supply to terminals 112 and 114, with the polarity of the power supply being as indicated at terminals 112 and 114, will be energized while relay 100 is awaiting a signal from sensor means 160.

Transistor 200 is a power transistor which is selected to carry the current and voltage demanded by the connected apparatus. As will hereinafter be explained, transistor 200, instead of being a power transistor, may be a less costly transistor which is used to provide gating signals for one or more high power devices which are less costly than power transistors for comparable ratings, such as silicon controlled rectifiers.

Transistor 192 may drive more than one power transistor 200, if more than one circuit is desired which provides a closed contact in the absence of a fault in the transformer.

When a fault occurs within the casing of the transformer and transistor 132 switches to its conductive state, its output is amplified by amplifier 106, which may include an NPN junction type transistor 210 having base, collector and emitter electrodes b, c and e, respectively. The base electrode b of transistor 210 is connected to the collector electrode 0 of transistor 132 through resistor 211, its emitter electrode e is connected to the voltage divider comprising resistors 212 and 214, which are connected between buses 134 and 140, and its collector electrode c is connected to bus 134 through resistor 216. Transistor 210 also isolates the flip-flop circuit 102 from being affected by leakage current from static switching means 110.

Static switching means 110, which may be a PNP junction type transistor 220 having base, collector and emitter electrodes b, c and e, respectively, has its base electrode b connected to the collector electrode 0 of transistor 210, its collector electrode 0 connected to bus through resistor 222, and its emitter electrode 2 connected to the voltage divider which includes resistors 224 and 226, which are connected between buses 134 and 140.

In this instance, instead of making transistor 220 a power transistor capable of handling the current of the protective devices, transistor 220 is used to provide switching signals for switching means 118, which may be a silicon controlled rectifier, a silicon controlled switch, or any other suitable static switching means.

Switching means 118 is shown with two static switching devices 230 and 232, but any number may be connected in parallel, in order to provide the desired number of normally open contacts. Device 230, which is shown as a semiconductor controlled rectifier, such as a silicon controlled rectifier, has anode, cathode and gate electrodes a, c and g, respectively, with its anode electrode being connected to terminal 120, its cathode electrode being connected to terminal 122 and to ground 138', and its gate electrode being connected to the collector electrode 0 of transistor 110. Device 232, which is also shown as a silicon controlled rectifier, has anode, cathode and gate electrodes a, c and g, respectively, with its anode electrode a being connected to terminal 124, its cathode electrode c being connected to terminal 122 and ground 138', and its gate electrode g being connected to the collector electrode 0 of transistor 110. Thus, when sensor means provides a signal which switches the flip-flop circuit into providing an output from transistor 132, which is amplified by transistor 210 and applied to transistor 220, to cause it to switch to its conductive state, a signal will be continuously applied to the gate electrodes g of switching devices 230 and 232. Since the gate signal is continuous, the electrical potential which is serially connected with each of the switching devices, to terminals 120 and 122, and to terminals 122 and 124, along with a protective device in each series circuit, may be an alternating potential, and switching devices 230 and 232 will allow the positive half cycles of the current to flow through its associated protective device. If the electrical potential connected to each of the switching devices 230 and 232 is a direct current potential, the direct current potential should be connected according to the polarities marked on terminals 120, 122 and 124, in FIG. 2. Also, if the electrical potential is a direct current potential, the reset means should contain switching means which momentarily interrupts the circuits of the switching devices 230 and 232, in order to allow their gate electrodes to regain control.

Relay 100 thus has two stable operating conditions, with the first operating condition being the normal condition, triggered into this condition by reset means 180, and the second operating condition being the alarm condition, triggered into this condition by a signal from sensor means 160. Relay 100 has static switches, some of which are conductive during the normal condition, and some of which are nonconductive, with the static switches switching to their opposite operating modes when the relay is driven into its alarm condition.

In the embodiment of the invention shown in FIG. 2, the signal from sensor means 160, which triggers relay 100, is the closing of mechanical contacts. The potential for triggering the relay action is derived from the direct current source associated with the relay which is indicated as battery 136. The relay circuit may easily be modified for use with an electronic transducer sensor, which supplies a direct current potential upon a sudden increase of pressure, by connecting a transducer 233, shown in dotted outline in FIG. 2, to reverse bias transistor 130, when the I transducer 233 provides an output potential. The mechanical sensor means 160, in this embodiment, would be eliminated.

Thus, in the complete operation of the protective systern, suitable sensor means will be disposed to be responsive to the pressure within the casing 16 of transformer 10, either in the air space above the liquid dielectric level 18, as shown in FIG. 1, or in the liquid itself, as shown in FIG. 1A. The sensor means is connected to the static relay 100, which is disposed in a suitable control panel 32, and relay 100 has its normally open switching circuits and normally closed switching circuits connected to appropriate alarm and protective means. Reset means 180 would be manually actuated to set the relay in its normal operating condition, in which transistors 130, 192, and 200 are conductive, and transistors 132, 210 and 220 are non-conductive. Thus, contacts 112 and 114 may be connected to an alarm circuit 116, which is energized only when transistor 200 is conductive, which is during the normal operating period of the relay. Contacts 112 and 114rfiay be connected to an alarm circuit which will automatically indicate a malfunction of relay 100, in those instances where alarm circuit 116 provides an alarm, without operation of the protective apparatus 126. Contacts 120, 122 and 124 may be connected to protective and alarm apparatus 126, which may be a circuit breaker, alarm, and/or other apparatus which provides the desired functions.

The protective system shown and described herein is substantially faster than similar systems of the prior art, with any time delay between the transformer fault and the operation of the protective devices being almost wholly due to the time lag in the sensor means. If the sensor means is an electronic transducer, instead of a mechanical bellows type arrangement, the time from fault detection to operation of the protective apparatus could be reduced still more. Further, relay 100 is rugged, compact, unaffected by thermal changes within the operating temperature range of normal transformer operation, and is not subject to contact bounce, or to false tripping due to vibration and shock, as there are no moving contacts subject to vibration.

Since numerous changes may be made in the abovedescribed apparatus and different embodiments of the invention may be made without departing from the spirit thereof, it isintended that all matter contained in the foregoing description or shown in the accompanying drawings, shall be interpreted as illustrative, and not in a limiting sense.

We claim as our invention:.

1. An electrical transformer comprising a magnetic core,

electrical windings disposed in inductive relation with said magnetic core,

a casing,

said electrical windings and magnetic core being disposed within said casing,

fluid insulating means disposed in said casing,

sensing means for sensing rate of change of pressure,

said sensing means being disposed to sense the pressure within said casing, said sensing means providing a signal when the rate of change of pressure in said casing exceeds a predetermined magnitude,

reset means, and solid state relay means, said solid state relay means including a bistable flip-flop circuit having first and second output terminals, first and second amplifier means connected to said first and second output terminals, respectively, and first and second static switching means connected to said first and second amplifier means, respectively,

said reset means being connected to said flip-flop circuit, causing said flip-flop circuit to switch its output from its second to its first output terminal when said reset means is activated, to switch said first static switching means to its conductive state and said second static switching means to its non-conductive state, said sensing means being connected to said flip-flop circuit, causing said flip-flop circuit to switch its output from its first to its second output terminal when said sensing means provides a signal, to switch said second static switching means to its conductive state and said first static switching means to its non-conductive state, the switching of said first and second static switching means in response to said flip-flop circuit providing normally open and normally closed static contacts adapted for connection to protective apparatus.

2. The electrical transformer of claim 1 wherein said fluid insulating means is an insulating liquid disposed to a predetermined level in said casing, which provides a predetermined air space above said insulating liquid, and said sensing means is disposed to sense the pressure in said air space.

3. The electrical transformer of claim 2 wherein the signal provided by said sensing means is the closing of a mechanical contact, which triggers said bistable flip-flop circuit to provide an output signal at its second output terminal.

4. The electrical transformer of claim 1 wherein said fluid insulating means is an insulating liquid and said 1senscilng means is disposed to sense the pressure within said qur 5. The electrical transformer of claim 4 wherein the signal provided by said sensing means is an electrical potential, which triggers said bistable flip-flop circuit to provide an output signal at its second output terminal.

US. Cl. X.R. 

